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How to Choose the Right Magnet for a Magnetic Box
Choosing a magnet is mostly a question of constraints, not strength. This guide covers neodymium grades from N35 to N52, why diameter and thickness are limited by the board rather than by force requirements, how magnet count and position should follow where the lid deflects, the six values every RFQ line item needs, and how to verify what actually arrived.
What this guide covers
Before selecting a magnet, it is worth being clear about its limits. Upgrading the magnet is the first move most projects make when a closure feels weak, and it frequently does nothing.
Magnetic attraction falls off steeply with distance. If something is holding the two magnet faces apart — an air gap from board flex, a proud magnet, or hinge spring-back — the extra strength bought by a higher grade is largely spent across that gap rather than felt at the lid.
Sequence matters more than grade
Eliminate the air gap, fix hinge spring-back, then size the magnet for what remains. Doing this in the opposite order means paying for strength that cannot be delivered.
The grade number in a neodymium designation relates to its maximum energy product, expressed in MGOe. Higher numbers mean more magnetic output for a given volume — and higher cost, at a rate that steepens at the top end.
Table 1: Relative output of common grades. *Relative order of magnitude; actual pull force depends on size, coating and test conditions.
| Common grade | Relative output* | Practical position |
|---|---|---|
| N35 | Baseline | Adequate for light lids; lowest cost |
| N38 | Slightly above baseline | Common default for rigid gift boxes |
| N42 | Moderately above baseline | Good balance where the lid has real mass |
| N45 | Higher | Justified only when space constrains magnet size |
| N48–N52 | Highest commonly available | Rarely economic in packaging applications |
Beyond output, two properties deserve attention in packaging:
For a given magnet volume, pull force is roughly a function of the contact area and the magnetic circuit. In a packaging context the practical reading is this: diameter and thickness both matter, but they are constrained by the board.
Table 2: Three constraints on size selection — in most cases the board, not the magnetism, sets the upper limit.
| Parameter | Constraint | Consequence of over-sizing |
|---|---|---|
| Diameter | Must sit within the lid wall without breaking through the wrap | Visible bump under the wrapping paper; wrap may wear through |
| Thickness | Must not exceed the board thickness available in the pocket | Magnet sits proud → permanent local gap → lid does not close flush |
| Count | Limited by panel area and by visual symmetry | Diminishing returns once the closing force is sufficient |
From the factory floor
A magnet thicker than the board is the most common assembly defect: the magnet protrudes from the board surface and forces a gap between the lid and base. It is often visually hard to notice, yet it causes complaints that "the lid never closes tightly.
When selecting, verify both magnet thickness and board thickness, and leave at least 0.2–0.3 mm of recess margin for the wrap paper and adhesive.
Count and position should follow where the lid actually deflects, not where symmetry suggests. On most lid-and-base formats two magnets are standard, positioned within the lid panel near the opening edge.
A magnet specification that can be sourced consistently and verified on receipt needs six values. Writing these in your RFQ removes most of the ambiguity that later becomes a dispute.
| Material | Neodymium (NdFeB), sintered, unless a substitute is agreed |
| Grade | e.g. N42 — state it explicitly; it is commonly omitted |
| Dimensions | Diameter × thickness in mm, e.g. 10 × 2 mm |
| Coating | Commonly Ni-Cu-Ni plating; state if corrosion resistance is required |
| Quantity | Per box, with position noted on the drawing |
| Performance check | The verification method — see below |
Note the last item: specifying the part without specifying how it is checked leaves you unable to reject a substitution. If your supplier changes grade without notice — which happens when stock is short — you want to have a test that reveals it.
Two practical checks, neither requiring laboratory equipment:
Keeping one approved reference unit in your office is the most reliable and least expensive control available. Every subsequent batch can be held next to it and felt, opened and measured — which catches both specification drift and process drift.
For the force-testing method itself, see how to test magnetic box closure strength.
Selecting a magnet is mostly about constraints rather than strength. Fix the air gap and hinge spring-back first, then size grade and dimensions within what the board can physically accept, add magnets rather than extreme grades when more force is needed, and specify six values so that a substitution can be detected on receipt.
FAQ
What magnet grade should I use for a rigid gift box?
N38 or N42 covers most applications. Choose based on lid mass and the required feel rather than defaulting to the highest grade, since cost rises faster than perceived benefit at the top end.
Can I just use a stronger magnet if the lid feels weak?
Only if there is no air gap. If the faces are being held apart by board flex or spring-back, extra strength is spent across that gap and the lid will still feel weak.
How thick can the magnet be?
It must sit below the board surface with room remaining for wrap and adhesive. Exceeding available board depth is a common cause of lids that will not close flush.
Will magnets lose strength over time?
Neodymium magnets have very low self-demagnetisation in normal use. Apparent loss is usually mechanical — corrosion, or migration out of the pocket — rather than a loss of magnetisation.
Do I need to worry about temperature in shipping?
For standard ambient distribution, no. Elevated temperature reduces performance, so if goods are stored or shipped hot, discuss temperature-rated grades with your supplier.
How do I know the supplier used the grade I specified?
Specify the grade in writing and verify functionally against a retained reference unit. Dimensional checks alone will not reveal a grade substitution.
Specifying magnets for a new box design?
Send the lid construction and board thickness — we can recommend a grade, dimension and position that fits what the board can physically accept, rather than the other way round.
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